The present invention generally relates to the production of silicon bodies, and specifically, to a process for controllably etching silicon dioxide from the surface of such bodies. The invention particularly relates, in a preferred embodiment, to a process for cleaning the surface of single crystal silicon bodies, such as wafers.
Microelectronic semiconductor devices are typically fabricated on single crystal silicon wafers produced from single crystal ingots commercially grown by the well-known Czochralski method. The silicon wafers are typically prepared through a sequence of steps in which the ingot is trimmed to a precise diameter, flattened along one or more sides to identify crystallographic orientation, etched to remove mechanical defects, and then sliced along a specific orientation into unfinished wafers. The unfinished wafers are precisely shaped through further fabrication steps including edge-rounding, lapping, etching and polishing.
The yield and performance characteristics of a microelectronic device are significantly impacted by the level of surface contamination on the silicon wafers. Generally, both organic and metallic contaminants may become physically or chemically attached to the surface, usually due to the fabrication steps described above and the associated handling of the wafers. This concern has become more acute as device geometry has become increasingly smaller. Accordingly, the removal of such contamination is necessary between wafer fabrication and shaping steps, after polishing, and also during device fabrication processes.
A variety of chemical processes for cleaning contaminants from the surface of silicon bodies are known in the art. The "RCA method" is a well known commercial method for cleaning silicon wafers. In this method, the wafer is sequentially exposed to two different chemical solutions, typically referred to as "Standard Cleaning 1" (SC1) and "Standard Cleaning 2" (SC2) solutions. See F. Shimura, Semiconductor Silicon Crystal Technology, p.189, Academic Press (San Diego, Calif., 1989).
The SC1 solution consists of ammonium hydroxide, hydrogen peroxide and water in respective ratios ranging from about 1:2:7 to about 1:1:5 parts by volume of commercially available reagents, typically supplied as 28-30 wt % NH.sub.4 OH in water and as 30-35 wt % H.sub.2 O.sub.2 in water. The ratios of ammonium hydroxide and hydrogen peroxide are expressed relative to water, but independent of each other. That is, in the typical SC1 solution, the ratio of NH.sub.4 OH:H.sub.2 O ranges from about 1:7 to about 1:5, the ratio of H.sub.2 O.sub.2 :H.sub.2 O ranges from about 2:7 to about 1:5, and both of these ratios (NH.sub.4 OH:H.sub.2 O and H.sub.2 O.sub.2 :H.sub.2 O) are independent of each other. Expressed in parts per million by weight relative to the amount of water, the concentration of NH.sub.4 OH in SC1 typically ranges from about 35,000 ppm to 50,000 ppm. SC1 solutions with ratios of reagents of about 1:4:20 NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O (approximately 13,300 ppm NH.sub.4 OH) or of about 1:20:100 (approximately 2,800 ppm NH.sub.4 OH) are also known in the art. Meuris et al., A New Cleaning Concept for Particle and Metal Removal on Si Surfaces, Electrochem. Soc. Proc. Vol. 94-7, pp. 15-24 (1994).
The ammonium hydroxide and hydrogen peroxide in the SC1 solution simultaneously etch the silicon dioxide layer and oxidize the underlying silicon substrate, respectively. As it etches, the ammonium hydroxide removes organic contaminants and also complexes metallic contaminants such as copper, gold, nickel, cobalt and cadmium. However, an alkaline etchant, such as NH.sub.4 OH, would, if it came in contact with the underlying silicon substrate, etch the silicon in an anisotropic, crystal-orientation dependent manner and severely roughen the surface. The hydrogen peroxide in the SC1 solution prevents the ammonium hydroxide from etching the underlying silicon substrate. The rate of etching SiO.sub.2 in the SC1 cleaning system is typically about 1-5 .ANG./min. However, etch rates of conventional SC1 solutions vary considerably with time, leading to non-uniform cleaning of silicon bodies. Additionally, the combination of the high basicity with high oxidation potential in the SC1 solution results in the precipitation of iron and aluminum oxides.
The second step of the RCA process, treatment with the SC2 solution, is primarily a remedial step aimed at the removal of the iron and aluminum oxides which arise during the SC1 step. The SC2 solution typically comprises hydrochloric acid, hydrogen peroxide and water in respective ratios ranging from 1:2:8 to 1:1:6 parts by volume of commercially available reagents, typically supplied as 28-30 wt % HCl in water and as 30-35 wt % H.sub.2 O.sub.2 in water. The ratios of hydrogen chloride and hydrogen peroxide are expressed relative to water, but independently of each other. The SC2 solution forms soluble metallic complexes with alkali and transition metals.
Alternative approaches have been tried to avoid or reduce the problem of iron and aluminum precipitates. One approach was to use ultrapure reagents for the SC1 solution. However, even using reagents in which the concentration of iron and aluminum is less than 10 ppt, the SC2 step was required. Because most of the metallic impurities in the SC1 bath come from hydrogen peroxide, another approach was to use other oxidants, such as ozone, which are available without metallic impurities. However, attempts to replace the hydrogen peroxide with ozone were unsuccessful because ozone destroyed the ammonium hydroxide.
Improvements and variations of the RCA method are also known in the art. For example, complete removal of the naturally occurring silicon dioxide layer overlying the silicon substrate is believed to facilitate surface cleaning. To this end, brief etching of the silicon wafer in a very dilute, high-purity hydrofluoric acid (HF) solution may be employed between the SC1 and SC2 steps of the RCA method. Beyer and Kastl, Impact of Deionized Water Rinses on Silicon Surface Cleaning, J. Electrochem. Soc. 129, 1027-29 (1982). Another improvement relates to the use of megasonics with the SC1 and SC2 baths, as reported in U.S. Pat. No. 4,804,007 to Bran.
The use of megasonic cleaners for other, non-SC1 cleaning schemes is reported, for example, in U.S. Pat. No. 3,893,869 to Mayer et al., which discloses the use of megasonic frequency sound energy to facilitate loosening of particles on the surfaces of semiconductor wafers. Further advances in megasonic cleaning apparatus and applications have been made, as exemplified by U.S. Pat. Nos. 4,804,007 and 5,286,657 to Bran and U.S. Pat. No. 5,279,316 to Miranda relating to an improved megasonic transducer array, a high-intensity single-wafer system and a single-bath/multi-process system, respectively. Despite these improvements, the level of impurities and variation in etch rates associated with conventional SC1/SC2 silicon wafer cleaning systems remain problematic.